UPDATE: A little more explanation here. I am not saying that there will not be any business applications using the Metro interface or consumer applications using the desktop interface. I actually belive that we will be seeing lots of split personality LOB applications where a read only or quick entry screen will use the Metro interface while the main application is still using the Desktop interface. However, I am saying that I think most LOB applications will not lend themselves well to Metro.

My current thinking is based around information density. I think the Metro UI is extemely good at presenting a relatively small amount of data to the user in a very easy to consume way. For example, by glancing at my phone I can quickly understand everything from the weather to how many emails I have. The desktop metaphor has never been very good at this kind of thing. On the other hand, the "classic" desktop UIs are extremely good at presenting extremely large amounts of data to the user in a understandable way. The LOB applications I work on every day would never work in a Metro UI. There are other parts that are screaming to be put into Metro so that they can be used in the field on a tablet. One of the big challenges we will be facing is how to write composite applications in Metro and Desktop UIs that do not drive our users crazy flipping back and forh between UI types. I have coined a term for this "UX Thunking". I hope to see the term spread.